152 SOUTH AFRICAN BOTANY 
134. Contact.—Certain organs of a plant are sensitive 
to contact, e.g. root-tips, tendrils and some stems. If 
a growing root encounters an obstacle, such as a stone, 
it will curve away from it, continuing its downward 
course when it has curved round the stone. This pro- 
perty is of great use to roots penetrating the soil, and 
can easily be shown experimentally. 
135. Experiment 28.—Fix a few stones by means of 
sealing wax to the sides of a glass funnel, then line the 
funnel with blotting-paper and fill it with sawdust. 
Place some soaked seeds between the blotting-paper 
and the glass, above the obstacles. As the seeds germi- 
nate, the roots will grow downwards along the sides 
of the funnel, till they reach the obstacles, when they 
will curve round them. The stones must be quite 
small, 
136. Tendrils.—The tip of a young tendril grows 
around in a spiral until it comes in contact with some 
support, it then becomes concave at the point of con- 
tact, and if the support is suitable, the stimulus is trans- 
mitted to other parts of the tendril and it twines round 
the support. At the same time the part of the tendril 
below the point of attachment becomes spirally coiled, 
thus drawing the stem close up to the support. Since 
both ends of the tendril are fixed before coiling begins, 
the spiral has to be reversed at some point ; if it is right- 
handed in the upper part, it will be left-handed below. 
This coiling of the tendril causes it to act like a spring, 
and diminishes the effects of shock or strain. 
The stigma of Bignonia is sensitive to contact, the 
two lobes close when touched, and do not open for some 
